Clipperships of the Cosmos: Solar Sails Move from Theory to Fact

One day vast solar sails pushed along by rays of sunlight may carry payloads between the planets. Tracy McConnell explains how this romantic vision is slowly becoming a fact.

Pioneering Solar Sail: Artist’s impression of Nanosail-D after deployment above Earth’s atmosphere (image credit: NASA)

The concept of sailing gracefully through the stars is a provocative one that has been around for longer than you may think. Over 400 years ago, German astronomer Johannes Kepler (1571-1630) was observing the tails of a comet being blown by an apparent solar breeze and suggested that, “ships and sails proper for heavenly air should be fashioned” in order to glide through space. Many years later, Scottish scientist James Clerk Maxwell (1831-79) demonstrated that sunlight exerts pressure on a reflective material, caused by the photons (particles of light) within the light bouncing off the surface. The force exerted is tiny but continuous, and over time should allow a space craft to build up a huge velocity. This is the basic idea behind solar sails, however the practical application of this theory hasn’t always run a smooth course.

In 1960, the large balloon-like satellite Echo-1 felt these solar pressure effects loudly and clearly. “Photon pressure played orbital soccer with the Echo-1 thin-film balloon in orbit…. The shards were flung far and wide by sunlight.” (Image credit: NASA)

NASA launched Echo 1 and Echo 2 in the 1960’s, these balloon satellites were settled into low Earth orbit and designed to reflect radio and telephone signals. Both of these balloons experienced a certain amount of solar pressure, acting as solar sails, due to their large size, however this reaction was coincidental to their purpose. In 1974 NASA used solar radiation pressure deliberately, although not as a form of propulsion. The Mariner 10 spacecraft ran low on altitude control gas while on its mission to Mercury. The controllers angled Mariner’s solar arrays into the Sun, and used the radiation pressure to boost the altitude of the spacecraft. This was not what the solar arrays were designed for, and although the amount of pressure present was very small however it was a triumphant example of the principle of solar sail propulsion at work.

In 1993 the Russian Space Agency launched Znamya 2, originally to be a solar sail design that was instead used to demonstrate the principle of using a space mirror to illuminate the Earth. The 20 m (65.6 feet) spinning mirror was supposed to re-direct a beam of sunlight towards the surface of the Earth. The mirror deployed successfully on 4 February 1993, and when illuminated, produced a 5km (3.1 mile) bright spot on the ground almost as bright as the full moon, which swept across Europe at a speed of 8 km/s (5 miles/s). Sadly this amazing piece of technology burned up on re-entry several hours after deployment, and its successor was damaged during deployment.

So we now know that satellite arrays experience a small amount of solar radiation pressure due to their size and reflectivity. India sought to counteract the torque caused by this solar pressure on the solar arrays that powered their telecommunication satellites, INSAT 2A in 1992 and INSAT 3A in 2003, by installing solar sails on the opposite side of the satellite.

All of the instances of solar sailing that we have looked at so far have been coincidental or secondary to the main propulsion of the spacecraft. There was a mission planned in conjunction with Russia and the Planetary Society in 2005 which had great promise. Cosmos 1 was to be the first solar sail spacecraft, which would rely exclusively on solar pressure to control the crafts orbit. Unfortunately the spacecraft was lost when the launch vehicle failed.

All of these experiences taught us valuable lessons about how to go about such a project, not only did the design of the sail need to be sound but so did the means of launching it and deploying it. All of these lessons came together on 21 May 2010 when the Japanese Aerospace Exploration Agency, JAXA, launched the Interplanetary Kite-craft Accelerated by Radiation Of the Sun, IKAROS, solar sail into space. This 200 m² (2100 ft²) membrane is attached to a small disc spacecraft. It successfully deployed on 10 June 2010 at a distance of 7.7 million km (4.8 million miles) from Earth as the spacecraft made its way to Venus. The tricky deployment was achieved by the central body of the craft rotating and using the centrifugal force to pull the four weighted corners of the square sail taught. As well as having the ultra thin highly reflective surface to “catch” the pressure from the solar photons, the craft also had thin film solar cells in order to generate solar power. The solar sail had confirmed acceleration due to solar radiation by 9 July 2010. The IKAROS project has been in full operation for nine months, and it represents a true pioneer in the field of solar sail propulsion technology.

The Ikaros mission profile (Image credit: JAXA)

Following this solar sail triumph, NASA launched its own experimental solar sail craft into low Earth orbit on 20 November 2010, Nanosail-D (aka Nanosail-D2 to separate it from the previous failed mission Nanosail-D). However not all went as planned. The sail was supposed to eject from the rocket on 6 December 2010 but there was some fault in the system. Thankfully the craft self deployed its 30.5 m² (100 ft²) weeks later on 20 January 2011. The Nanosail-D should remain in low Earth orbit for 70 – 120 days. Its main purpose is to demonstrate and study the solar sail deployment technology in an attempt to improve the design of the sail itself as well as looking at alternative ways of “bringing home” old retired satellites and other space junk. You can follow its mission on twitter at http://twitter.com/nanosaild The solar revolution is on the up, with yet another contender, The Planetary Society’s Lightsail-1 on NASA’s shortlist to piggy-back onto an upcoming mission, that might make three successful solar sail missions in just one year!

Why are these missions so important? Well, currently the cost of travelling into space is huge in part because of the amount of fuel required for the journeys, and granted such expenditure will probably have to remain in order to get any spacecraft beyond the effects of our planets gravity. But open space? Travelling to and from planets and even to and from other stars?

The principle of any rocket engine requires that a rocket burn for a few minutes, followed by the craft cruising at a constant velocity. The overwhelming benefit of a solar sail is that although slow to get going, it accelerates constantly, and could potentially reach speeds much greater than those of a rocket propelled spacecraft.

According to www.planetary.org the Planetary Society’s website, at an acceleration of 1 millimetre per second, a solar sail would increase its speed by approximately 310 km/h (195 mph) after 1 day, and would travel 7500 kilometres (4700 miles). After 12 days it would have increased its speed to 3700 km/h (2300 mph). Such speeds and distances would allow much more accessible travel between our planets, but perhaps not other stars. We may need to think outside a whole new box for another type of propulsion that would achieve that journey. But that’s a story for another time.

(Article by Tracy McConnell)